1. Field of the Invention
The present invention relates to method for manufacturing semiconductor device, and in particular to a improved method for manufacturing semiconductor device, wherein a cleaning process of a buffer layer is performed prior to a formation of a nitride film so that a decrease in a deposition thickness of the nitride film is prevented even when the time between the formation of the buffer layer and the formation of the nitride film is long to improve a reliability of the device.
2. Description of the Background Art
As the size of a semiconductor device is reduced, the thickness of the film deposited in the device is also reduced. When a nitride film for gate spacer is deposited directly on a semiconductor substrate, characteristic of the device is degraded due to a stress of the nitride film on the semiconductor substrate during subsequent thermal processes. In order to reduce the stress, a buffer layer between the semiconductor substrate and the nitride film has been proposed. However, when the period between the times of the deposition of the buffer layer and the nitride film is increased, the deposition thickness of the nitride film is drastically decreased.
In addition, when the length of a channel region is decreased due to the reduction in size of the device, the dose of ion-implant for adjusting threshold voltage must be increased to prevent decrease of cell threshold voltage due to a short channel effect. The increase in the dose to adjust threshold voltage not only results in increase in resistance and a coulomb fail but also increase in write time and an increase in leakage current due to reduction in high concentration source/drain junction region and depletion region. Moreover, a refresh time is also decreased.